I'm trying to explain to a friend on msn what mixins are.  Can you
provide some good examples, that would be difficult otherwise?

-Joel

janderson wrote:
> I'm trying to explain to a friend on msn what mixins are.  Can you
> provide some good examples, that would be difficult otherwise?
> 
> -Joel

What about implementing multiple inheritance in a language that does not have multiple inheritance? (interface + mixin as default implementation)

Lutger wrote:
> janderson wrote:
>> I'm trying to explain to a friend on msn what mixins are.  Can you
>> provide some good examples, that would be difficult otherwise?
>>
>> -Joel
> 
> What about implementing multiple inheritance in a language that does not have multiple inheritance? (interface + mixin as default implementation)

Thanks for your reply.

Well coming from C++ this would not make much sense.  Where would you use them in C++?  Where would you use them in D?  Are there any good D examples that show the power of mixins in a simple way that would be more ugly another way?

Cheers.
-Joel

janderson escribió:
> Lutger wrote:
>> janderson wrote:
>>> I'm trying to explain to a friend on msn what mixins are.  Can you
>>> provide some good examples, that would be difficult otherwise?
>>>
>>> -Joel
>>
>> What about implementing multiple inheritance in a language that does not have multiple inheritance? (interface + mixin as default implementation)
> 
> Thanks for your reply.
> 
> Well coming from C++ this would not make much sense.  Where would you use them in C++?  Where would you use them in D?  Are there any good D examples that show the power of mixins in a simple way that would be more ugly another way?
> 
> Cheers.
> -Joel

The singleton pattern is a nice and real world example of some use for mixins:

# template Singleton() {
#
#    protected static typeof(this) _instance;
#    protected this() { }
#
#    public static typeof(this) instance() {
#        if (!_instance) {
#            _instance = new typeof(this)();
#        }
#        return _instance;
#    }
#
# }

And you use it like this:

# class MySingletonClass {
#
#     mixin Singleton!();
#
# }

I believe mixins are just to save you from writing bolierplate code, I don't know if there are other uses...

Ary

Ary Manzana wrote:
> janderson escribió:
>> Lutger wrote:
>>> janderson wrote:
>>>> I'm trying to explain to a friend on msn what mixins are.  Can you
>>>> provide some good examples, that would be difficult otherwise?
>>>>
>>>> -Joel
>>>
>>> What about implementing multiple inheritance in a language that does not have multiple inheritance? (interface + mixin as default implementation)
>>
>> Thanks for your reply.
>>
>> Well coming from C++ this would not make much sense.  Where would you use them in C++?  Where would you use them in D?  Are there any good D examples that show the power of mixins in a simple way that would be more ugly another way?
>>
>> Cheers.
>> -Joel
> 
> The singleton pattern is a nice and real world example of some use for mixins:
> 
> # template Singleton() {
> #
> #    protected static typeof(this) _instance;
> #    protected this() { }
> #
> #    public static typeof(this) instance() {
> #        if (!_instance) {
> #            _instance = new typeof(this)();
> #        }
> #        return _instance;
> #    }
> #
> # }
> 
> And you use it like this:
> 
> # class MySingletonClass {
> #
> #     mixin Singleton!();
> #
> # }
> 
> I believe mixins are just to save you from writing bolierplate code, I don't know if there are other uses...
> 
> Ary

With the use of tuples and static foreach, they can also be used to insert generated code into a scope. this is handy if the generated code needs to access user defined code

I just posed about an example of this (and a lot more template work) over in digitalmars.D.announce "D Template based paser generator in 137 loc".

janderson wrote:
> Lutger wrote:
>> janderson wrote:
>>> I'm trying to explain to a friend on msn what mixins are.  Can you
>>> provide some good examples, that would be difficult otherwise?
>>>
>>> -Joel
>>
>> What about implementing multiple inheritance in a language that does not have multiple inheritance? (interface + mixin as default implementation)
> 
> Thanks for your reply.
> 
> Well coming from C++ this would not make much sense.  Where would you use them in C++?  Where would you use them in D?  Are there any good D examples that show the power of mixins in a simple way that would be more ugly another way?
> 
> Cheers.
> -Joel

I think the Signals/Slots implementation in Phobos is a good example.

-- Chris Nicholson-Sauls

janderson wrote:
> I'm trying to explain to a friend on msn what mixins are.  Can you
> provide some good examples, that would be difficult otherwise?
> 
> -Joel

They're a convenient tool for inserting parametrized code into scopes - in some cases this can be partially accomplished in C++ through the (ab)use of multiple inheritance. For instance:

template <typename SomePolicy>
struct Foo : SomePolicy {
};

could be coded in D as:

struct Foo(SomePolicy) {
    mixin SomePolicy;
}

It doesn't seem like a big win in this case, but when the Policy must use the Foo struct for some reason, the C++ approach would be to specialize the policy on the struct's type, like
struct Foo : SomePolicy<Foo>

while in D, the mixin just knows about the struct's type because mixing it in means that it becomes a part of the struct's declaration, so for instance typeof(this) inside a mixin will work just like in the struct itself.

D further benefits from the fact that mixins can be easily named - it helps avoid naming conflicts.


--
Tomasz Stachowiak

Chris Nicholson-Sauls wrote:
> janderson wrote:
>> Lutger wrote:
>>> janderson wrote:
>>>> I'm trying to explain to a friend on msn what mixins are.  Can you
>>>> provide some good examples, that would be difficult otherwise?
>>>>
>>>> -Joel
>>>
>>> What about implementing multiple inheritance in a language that does not have multiple inheritance? (interface + mixin as default implementation)
>>
>> Thanks for your reply.
>>
>> Well coming from C++ this would not make much sense.  Where would you use them in C++?  Where would you use them in D?  Are there any good D examples that show the power of mixins in a simple way that would be more ugly another way?
>>
>> Cheers.
>> -Joel

Yes indeed, this is not a good example vis-a-vis C++.

Private inheritance in C++ is sometimes called mixin. So here, inheritance is used (but more restrictive) to emulate mixin's.

I find the seperation between (single) inheritance, interface and raw code injection better conceptually in D. In C++ it's all lumped together in inheritance, which makes it harder to distinguish intent.

There surely are good examples, I haven't used mixins myself in such a way that you are asking for.

Of course it is a unconvincing argument to make, but I would rather ask the question if there are any good examples where MI a la C++ is cleaner than interface and / or mixins because MI is so ugly it's on par with preproccesor macros and template template arguments.

> I think the Signals/Slots implementation in Phobos is a good example.
> 
> -- Chris Nicholson-Sauls

I think you can achieve this as easy with multiple inheritance in C++?

> >> Well coming from C++ this would not make much sense.  Where would you use them in C++?  Where would you use them in D?  Are there any good D examples that show the power of mixins in a simple way that would be more ugly another way?

Roughly speaking, anytime you see a multiline #define macro in C++ (especially with parameters), you would use a mixin in D.

That's only one sample use, but it should speak to the Cplusplusers.

Am 05.01.2007, 00:19 Uhr, schrieb Waldemar <waldemar@wa-ba.com>:

>> >> Well coming from C++ this would not make much sense.  Where would you
>> >> use them in C++?  Where would you use them in D?  Are there any good  
>> D
>> >> examples that show the power of mixins in a simple way that would be
>> >> more ugly another way?
>
> Roughly speaking, anytime you see a multiline #define macro in C++ (especially
> with parameters), you would use a mixin in D.
>
> That's only one sample use, but it should speak to the Cplusplusers.
>

Erm ... I don't think you can insert this line via a mixin into a certain scope:

' scope (failure) dosomething();

But, please, tell me I'm wrong, I would really need that :)

-mike

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